On Tue, Nov 9, 2021 at 1:10 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Tue, Nov 9, 2021 at 12:10 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Tue 09-11-21 12:02:37, Suren Baghdasaryan wrote: > > > On Tue, Nov 9, 2021 at 11:50 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > On Tue 09-11-21 11:37:06, Suren Baghdasaryan wrote: > > > > > On Tue, Nov 9, 2021 at 11:26 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > > > On Tue 09-11-21 11:01:02, Suren Baghdasaryan wrote: > > > > > > [...] > > > > > > > Discussing how the patch I want to post works for maple trees that > > > > > > > Matthew is working on, I've got a question: > > > > > > > > > > > > > > IIUC, according to Michal's post here: > > > > > > > https://lore.kernel.org/all/20170725154514.GN26723@xxxxxxxxxxxxxx, > > > > > > > unmap_vmas() can race with other mmap_lock read holders (including > > > > > > > oom_reap_task_mm()) with no issues. > > > > > > > Maple tree patchset requires rcu read lock or the mmap semaphore be > > > > > > > held (read or write side) when walking the tree, including inside > > > > > > > unmap_vmas(). When asked, he told me that he is not sure why it's > > > > > > > currently "safe" to walk the vma->vm_next list in unmap_vmas() while > > > > > > > another thread is reaping the mm. > > > > > > > Michal (or maybe someone else), could you please clarify why > > > > > > > unmap_vmas() can safely race with oom_reap_task_mm()? Or maybe my > > > > > > > understanding was wrong? > > > > > > > > > > > > I cannot really comment on the mapple tree part. But the existing > > > > > > synchronization between oom reaper and exit_mmap is based on > > > > > > - oom_reaper takes mmap_sem for reading > > > > > > - exit_mmap sets MMF_OOM_SKIP and takes the exclusive mmap_sem before > > > > > > unmap_vmas. > > > > > > > > > > > > The oom_reaper therefore can either unmap the address space if the lock > > > > > > is taken before exit_mmap or it would it would bale out on MMF_OOM_SKIP > > > > > > if it takes the lock afterwards. So the reaper cannot race with > > > > > > unmap_vmas. > > > > > > > > > > I see. So, it's the combination of MMF_OOM_SKIP and mmap_lock working > > > > > as a barrier which prevent them from racing with each other... > > > > > I wasn't sure how > > > > > https://lore.kernel.org/all/20170724072332.31903-1-mhocko@xxxxxxxxxx/ > > > > > was implementing this synchronization because it would take mmap_sem > > > > > write side after unmap_vmas() and IIUC there was no > > > > > "mmap_lock_write(); mmap_unlock_write();" sequence in exit_mmap at > > > > > that time. I'll need to checkout the old sources to figure this out. > > > > > > > > My memory is rather dimm but AFAIR the main problem was freeing page > > > > tables and freeing vmas not unmap_vmas. That one was no modifying the > > > > vma list. Essentially it was just a slightly modified madvise don't > > > > need. So that part was allowed to race with oom_reaper. > > > > > > So, both unmap_vmas and __oom_reap_task_mm do not modify vma list and > > > therefore can execute concurrently. That makes sense, thanks. > > > > Yes, those can run concurrently. One thing I completely forgot about is > > 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3") > > which is about interaction with the munlock. Agrh! This interaction with the munlock you mentioned requires us to take mmap_write_lock before munlock_vma_pages_all and that prevents __oom_reap_task_mm from running concurrently with unmap_vmas. The reapers would not be as effective as they are now after such a change :( > > Thanks for pointing it out. IIUC, ideally we want to get rid of all > these special cases and replace them with proper locking. If so, I'll > see what I can do here. > > > > > > Then I guess, if we want to be semantically correct in exit_mmap(), we > > > would have to take mmap_read_lock before unmap_vmas, then drop it and > > > take mmap_write_lock before free_pgtables. > > > > I think it would be just more straightforward to take the exclusive lock > > for the whole operation. > > Ok, but note that this will prevent concurrent memory reaping, so will > likely affect the speed at which memory is released during oom-kill. I > saw measurable difference when testing process_mrelease placing > mmap_write_lock before vs after unmap_vmas. If we take mmap_read_lock > before unmap_vmas and mmap_write_lock after it, then there won't be > such issue. You indicated that the speed of memory release should not > be the deciding factor here but I want to make it clear before > proceeding. > Thanks, > Suren. > > > -- > > Michal Hocko > > SUSE Labs
